Nonlinear VNAs Extend To 50 GHz

June 17, 2009
Available with nonlinear test capability as an option, this high-performance vector network analyzer series includes models with top frequencies of 13.5, 26.5, 43.5, and 50 GHz.

Vector network analyzers (VNAs) provide measured information about active and passive devices that can be invaluable when developing models for those devices. Recognizing the growing need for scattering-parameter (S-parameter) device characterization through millimeter-wave frequencies, Agilent Technologies has expanded its high-performance PNA-X series of VNAs to include a model with a frequency range of 10 MHz to 50 GHz (model N5245A). And for those not needing the full firepower, the PNA-X series offers the model N5241A for measurements from 10 MHz to 13.5 GHz, the model N5242 for measurements from 10 MHz to 26.5 GHz, and the model N5244A for applications from 10 MHz to 43.5 GHz. In addition, all four models are available in two-port and four-port versions and with nonlinear vector network analyzer (NVNA) measurement capability as an option, for developing models that include behavior under both linear and nonlinear operating conditions.

The PNA-X series VNAs (Fig. 1) are actually more like measurement systems than traditional microwave VNAs. The choices in frequency cover (see table) allow operators to tailor the needs of their applications to the measurement capabilities of a particular VNA. Each of the VNAs is designed for simple hookups with a device under test (DUT), providing a host of software-selectable measurement functions with one simple connection. The measurement flexibility of these analyzers is enhanced by the inclusion of front-panel jumpers for direct access to internal directional couplers and receivers, and source and receiver attenuators with 5-dB increments for better measurement optimization. For more advanced measurement systems using external test equipment tied to the PNA-X analyzers, the VNAs offer three sets of triggers to simplify synchronization. For example, a PNA-X can be synchronized with external pulse modulators and pulse generators for pulse measurement applications requiring complex timing.

All of the PNA-X analyzers feature internal mechanical switches for routing of stimulus signals and two internal test-signal sources along with an internal pulse modulator to create both CW and pulsed test signals. The second test signal source can also serve as a local oscillator (LO) for mixer and frequency-converter testing. In addition, the analyzers have a built-in signal combiner to simplify the creation of two-tone test signals for intermodulation distortion (IMD) testing and built-in bias tees to simplify biasing of amplifiers. The PNA-X series VNAs can perform a wide range of highfrequency measurements automatically, including source-corrected noise-figure measurements, gain compression measurements, measurements of deviation from linear phase with frequency, and true differential measurements on balanced components. They rely on several advanced calibration technologies to improve frequency-conversion ripple and group-delay accuracy during mixer and frequency-converter characterization.

All models offer as many as 32 measurement channels with 32,001 measurement points, with 10 markers per trace. All VNAs provide frequency resolution of 1 Hz with 1 PPM frequency accuracy and 0.05 PPM frequency stability at temperatures from -10 to +70C. The typical phase stability of the analyzers is an impressive 0.4 deg. to 26.5 GHz, 0.75 deg. to 43.5 GHz, and 0.80 deg. to 50 GHz.

For the flagship 50-GHz model N5245A VNA, the typical leveled power is +16 dBm to 26.5 GHz, +13 dBm to 43.5 GHz, and -1 dBm to 50 GHz. The specified power level accuracy is 2 dB to 26.5 GHz, 2 dB to 43.5 dBm, and 3 dB to 50 GHz, with typical power level accuracy of 0.47 dB to 26.5 GHz, 0.97 dB to 43.5 dBm, and 0.93 dB to 50 GHz. The analyzer can typically sweep across power ranges of 42 dB to 26.5 GHz, 40 dB to 43.5 GHz, and 26 dB to 50 GHz, and the power can be set with resolution of 0.01 dB. The analyzers perform a variety of different swept measurements, including linear, logarithmic, power, CW, and segment sweeps. The dynamic accuracy is typically better than 0.1 dB for test port power as low as -60 dBm and 1 dB for test port power as low as -100 dBm at frequencies to 50 GHz.

Internally generated test signals in the PNA-X analyzers are characterized by good spectral purity. Harmonics are typically -60 dBc while spurious levels are typically -39 dBc at 26.5 GHz, -31 dBc at 43.5 GHz, and -31 dBc to 50 GHz. The noise floor of the 50-GHz model is specified at -107 dBm measured in a 10-Hz intermediate-frequency (IF) bandwidth at 50 GHz, with typical performance of -113 dBm under the same measurement conditions. In comparison, the noise floor at 26.5 GHz is specified at -111 dBm and typically -114 dBm in a 10-Hz IF bandwidth, while the noise floor at 43.5 GHz is specified at -109 dBm and typically -113 dBm in a 10-Hz IF bandwidth. The 50-GHz VNA's receiver features a dynamic range of 129 dB while the overall system achieves a dynamic range of 126 dB. The receiver boasts a 0.1-dB compression point for an input level of +13 dBm to contribute to the wide dynamic range. For pulsed measurements, standard models include IF bandwidths to 5 MHz wide to support analysis of pulses as narrow as 250 ns.

The PNA-X VNAs feature a 10.4-in. touch-screen display to show measurement results and serve as part of the user interface. The analyzers can show a wide range of measurement results including linear S-parameters, amplifier gain compression, harmonic distortion, noise figure, and IMD. They work with traditional calibration techniques as well as several time-saving calibration approaches, such as an enhanced frequency-response calibration for high-power amplifier measurements and a quick short-open-loadthru (QSOLT) technique that reduces the number of calibration standards needed for multiport test setups compared to conventional SOLT approaches. In addition, when equipped with optional nonlinear-vector-network-analyzer (NVNA) measurement capability, each of the VNAs can perform measurements of nonlinear X-parameters, which are used to describe the nonlinear behavior of a DUT and can be incorporated into nonlinear device models (Fig. 2).

These high-performance VNAs are well suited for pulsed measurements, with built-in four-channel pulse generator and pulse modulator to create the required pulsed test signals. The analyzers also incorporate a 5-MHz IF bandwidth as a standard feature in wideband mode for measurement of pulses as narrow as 250 ns while also delivering 133 ns time resolution for pulse profile measurements. In narrowband mode, the analyzers can create stimulus signals as narrow as 33 ns (and typically 20 ns) with time resolution as fine as 33 ns (and typically 20 ns) when making pulse profile measurements.

Some of the keys in achieving high accuracy in pulsed measurements with the PNA-X analyzers are several unique signalprocessing approaches to reduce noise and increase the measurement dynamic range. One of the problems in processing narrow pulses is the need for a relatively wide IF filter bandwidth, even though the wide passband will raise the measurement noise floor, reduce the signal-to-noise ratio (SNR), and lower the measurement accuracy.

Most narrowband pulse measurement technology employs narrow IF filters to reject unwanted pulse spectral lines around the desired central spectral lines. The filter width may be between 0.1 and 1 percent of the pulse repetition frequency (PRF) and provide about 70 to 80 dB of rejection of wanted signals. Because the use of a narrowband filter also reduces the measurement speed, Agilent developed a patented technology called spectral nulling to make accurate pulse measurements without sacrificing processing speed. By properly aligning the nulls of a somewhat wider IF filter, it is possible to leave the desired signal while achieving as much as 120 dB rejection of unwanted signal components with reasonable measurement speed.

The PNA-X VNAs use a combination of hardware and software gating for pulse measurements. After passing through a hardware gate, IF signals are digitized and passed through a software gate. The software gate reduces the noise and crosstalk signals outside the hardware gate to significantly increase the measurement sensitivity. The advanced IF hardware helps to reduce the noise floor of the PNA-X analyzers by about 20 dB while the software gating lowers the noise floor by another 20 dB. Even for pulse duty cycles as low as 0.001 percent, the PNA-X analyzers provide a measurement dynamic range of 60 dB. The VNAs also feature extensive timedomain display capabilities to help when analyzing pulsed signals (Fig. 3).

The PNA-X VNAs also provide advanced technology to improve frequency- conversion and mixer measurements, such as a scalar mixer calibration approach that combines two-port calibration and power calibration to overcome in-band conversion-loss flatness issues. The technique involves making a source power calibration with a power meter to ensure that the source power is well characterized. That calibrated source is then used to calibrate the analyzer's measurement receivers. Then, a two-port calibration is performed to measure the other systematic errors that affect reflection and transmission measurements. This step removes mismatch errors that can occur during conversion loss or gain measurements, helping to reduce ripple. The approach can be implemented with an Agilent U2000 Series Universal Serial Bus (USB) power meter sensor so that an external power meter is not required.

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To improve the accuracy of mixer group-delay measurements, the PNA-X analyzers use a technique known as a vector mixer calibration. Group delay measurements require that the frequencies of the reference and test receivers are precisely the same. Because the two receivers will be used at the input and output of a frequency-translation component, an additional reference or calibration mixer is needed to ensure that the input signal to the reference receiver is the same frequency as the output signal to the measurement receiver.

The PNA-X analyzers are also well suited for amplifier measurements, including low-noise amplifiers (LNAs). Using a noise-figure-measurement approach known as the cold-source method, the analyzers eliminate the need for a noise source at the input of the amplifier under test. The analyzers use vector error correction to compensate for mismatch effects, and overcome noise-parameter-induced errors with the aid of a standard Agilent ECal electronic calibration module, which serves as a variable impedance tuner to vary the source match of the test system or each measurement frequency point. The approach allows the effects of switches, cables, fixtures, or probes to be removed from test results.

Perhaps the most powerful version of the new PNA-X VNAs is with the optional NVNA measurement capability and nonlinear X-parameters for device characterization and modeling (Fig. 2). X-parameters are a mathematical extension of S-parameters, but for nonlinear, large-signal conditions. The vector-corrected parameters can provide insight into a transistor, amplifier or other active device in its linear and nonlinear operating regions. In addition, the X-parameters can be readily extracted into Agilent's Advanced Design System (ADS). The capability of applying X-parameters to active devices through 50 GHz provides the possibility of developing valid nonlinear models of devices even through their harmonic regions. Agilent has also partnered with companies such as Maury Microwave and their load-pull tuners (Fig. 5) to provide the capability of measuring X-parameters under different load conditions. Agilent Technologies, Electronic Measurement Group, 5301 Stevens Creek Blvd., MS 54LAK, Santa Clara, CA 95052; (877) 424-4536, (408) 345-8886, FAX: (408) 345-8474, Internet:

About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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